1. Field of the Invention
This invention is directed to fabrication of high purity glass tubes that can be used for producing low loss optical fibers.
2. Discussion of the Known Art
Production of optical fiber to be used for data or other information transmission, is typically accomplished by drawing the fiber with the aid of gravity from an axial end of a cylindrical glass preform. The preform is supported vertically in the axial direction inside a furnace and is heated at temperatures typically about 2100 degrees C. or higher, thus causing a molten glass drop to form at the lower end of the preform from which the fiber is drawn. For low transmission loss, particularly at wavelengths around 0.95 microns (μ), it is known that the glass of which the preform is made should have an OH concentration of less than 1 ppb (1×10−9), with non-detectable transition metal content. Absent such high purity glass including, for example, F-doped, Ge-doped, and undoped silica glass preform tubes, certain desirable fiber transmission characteristics cannot be attained.
U.S. Pat. No. 5,182,052 (Jan. 26, 1993) discloses a method of making quartz glass tubes which, according to the patent, are of optical waveguide quality. A tubular mold is rotated about its axis, and a liquid binder is deposited on the inner wall of the rotating mold. A powder material including SiO2 is deposited over the binder, and the rotational speed of the mold is increased subjecting the powder material to centrifugal force and causing it to form a solid cylindrical “green body”. The green body is then removed from the mold, dried, and heated to form a glass tube.
A process for making glass tubes to be used as optical fiber preforms is disclosed in U.S. Pat. No. 4,191,545 (Mar. 4, 1980). Amorphous powdery particles are deposited via an oxidizing burner along the length of a cylindrical mandrel which is rotated about its axis. The particles form a porous intermediate tubular structure which is then cooled, removed from the mandrel and fused into a transparent glass tube. Additional material is then deposited on the inner portion of the tube as in a conventional modified chemical vapor deposition (MCVD) process, to yield a preform from which an optical fiber is drawn.
Optical fibers may also be produced by a so-called vapor phase axial deposition (VAD) method which does not at the outset require a high purity glass tube. See U.S. Pat. No. 4,062,665 (Dec. 13, 1977) all relevant portions of which are incorporated by reference. Basically, in VAD, a vertically suspended rod of silica acts as a “seed” wherein a core soot is initially deposited from a flame of a core soot burner onto a confronting axial end face of the rod. The rod is rotated about its axis and, as the soot deposition increases, the rod is moved vertically upward so as to maintain the axial end surface of the core soot at a constant level with respect to the core soot burner. As the rod continues to move upward a cladding soot burner has its flame directed onto the outer surface of the core soot, and the flame deposits a cladding soot which builds outward by a desired radial thickness beyond the core soot. A porous cylindrical soot structure of a desired length and refractive index profile is obtained, and the structure is then heated at a high temperature in a furnace for vitrification into a preform having a transparent glass core and cladding.
See also U.S. Pat. No. 5,281,248 (Jan. 25, 1994) and No. 6,131,415 (Oct. 17, 2000), and H. Murata, Recent Developments in Vapor Phase Axial Deposition, Journal of Lightwave Technology, vol. LT-4, No. 8 (August 1986) at pages 1026–33; all relevant portions of which are incorporated by reference. As far as is known, high purity glass tubes to be used in the production of optical fiber preforms have not been fabricated using a procedure the same as or similar to the VAD process described above.